Indian PSLV lofts IRNSS-1G to complete navigation constellation

India has launched a PSLV rocket carrying the IRNSS-1G navigation satellite. The launch, which took place early Thursday afternoon local time, completes deployment of the first-generation IRNSS constellation, giving India a fully-operational regional navigation system.ISRO Launch:

Thursday’s launch took place from the First Launch Pad of the Satish Dhawan Space Centre at 12:50 local time (07:20 UTC).

The PSLV took twenty minutes to place its payload into a low-apogee geosynchronous transfer orbit, from which the spacecraft will be able to manoeuvre to its operational orbit. It was the thirty-fifth flight of India’s Polar Satellite Launch Vehicle (PSLV) and the fifty-second launch overall to be made by India.

The objective of Thursday’s mission was to deploy the seventh member of the Indian Regional Navigation Satellite System (IRNSS), a constellation of satellites in geosynchronous and geostationary orbits intended to give Indian users, including the military, access to a dedicated satellite navigation system not dependent upon foreign governments or suppliers.

The space segment of the constellation consists of seven satellites, of which the one launching Thursday – IRNSS-1G – will be the last initial member. Four further satellites have been ordered to serve as ground spares.

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Single IRNSS satellites are stationed at three slots in geostationary orbit, with the remainder operating in pairs at two stations in an inclined geosynchronous orbit. IRNSS-1G will occupy a geostationary position at 129.5 degrees East – alongside IRNSS-1F at 34 degrees East and IRNSS-1C at 83 degrees East.

The geosynchronous satellites, which orbit with an inclination of 29 degrees to the equator, consist of IRNSS-1A and 1B at 55 degrees East and IRNSS-1D and 1E at 111.75 degrees East.

The IRNSS-1 satellites are identical 1,425-kilogram (3,142 lb) spacecraft developed by the Indian Space Research Organisation (ISRO) around the I-1K satellite bus.

Each satellite is powered by a pair of solar arrays, generating up to 1.66 kilowatts. Each spacecraft uses a liquid-propellant apogee motor for orbital manoeuvres, along with smaller thrusters for stationkeeping; these thrusters also form part of the satellites’ attitude control systems, along with reaction wheels and magnetorquers. Once on station, IRNSS-1G is expected to function for at least twelve years.

The seven first-generation satellites have been launched over a three-year period, starting with the deployment of IRNSS-1A in July 2013. ISRO has launched all of the satellites itself using the PSLV rocket.

The flight number for Thursday’s launch was PSLV C33, which saw the vehicle fly in its most powerful configuration, the PSLV-XL. This version of the PSLV was introduced in October 2008 with the launch of the Chandrayaan-1 lunar probe, and features more powerful solid rocket boosters than the standard PSLV, increasing the amount of payload it can carry into orbit.

The PSLV was not designed to launch geosynchronous missions and as such lacks sufficient performance to place IRNSS satellites into typical geosynchronous transfer orbits.

Instead, IRNSS-1G was dropped off in a lower subsynchronous transfer orbit. The satellite will perform additional orbit-raising manoeuvres to reach its operational orbit.

This is by design, and has been performed on all launches of the IRNSS programme; the performance trade-off allows the smaller, more established and reliable PSLV to be used; otherwise ISRO would have to rely upon either a foreign launch provider or its own larger Geosynchronous Satellite Launch Vehicle (GSLV) which has a chequered reliability record and a low flight rate.

Before the launch ISRO declared the allowable margins of error to be plus or minus five kilometres (3 miles) at perigee and 675 kilometres (419 miles, 364 nautical miles). In terms of inclination, the margin of error was plus or minus two tenths of a degree.

The PSLV departed from the First Launch Pad at the Satish Dhawan Space Centre, located on India’s Sriharikota Island. Formerly known as the Sriharikota High Altitude Range (SHAR), the facility was renamed in 2002 following the death of aerospace engineer and former ISRO chairman Satish Dhawan.

The centre has two active orbital launch complexes – designated the First and Second Launch Pads – both of which can be used by either the PSLV or GSLV. Typically, GSLV launches occur from the second pad, whose complex includes a vertical integration building to allow assembly of rockets away from the pad. PSLV launches will use whichever pad is available.

At the First pad rockets are vertically integrated, directly onto the pad, using a mobile service tower. This tower was rolled back away from PSLV C33 on Wednesday in preparation for Thursday’s liftoff.

The PSLV’s first stage and boosters burned solid propellant; using a compound based on hydroxyl-terminated polybutadiene (HTPB).

The first stage, or PS1, was augmented by six PS0M-XL boosters, four of which were ground-lit and the final two air-lit. First stage ignition occurred at the zero mark in the countdown, followed by the ground-lit boosters in two pairs 4.2 and 6.2 tenths of a second later. The air-lit boosters were ignited twenty-five seconds into the flight as the rocket ascended towards orbit.

Separation of the ground-lit boosters occurred seventy seconds after launch; the spent solids separating in pairs two tenths of a second apart. At 92 seconds mission elapsed time, the air-lit motors also separated.

Burnout and separation of the first stage came a minute and fifty seconds after liftoff, with a delay of two tenths of a second between separation and second stage ignition.

The second stage, or PS2, was powered by a Vikas engine – descended from the Viking engines developed by France for the Ariane family of rockets. The stage burned UH25 propellant – consisting of one part hydrazine hydrate and three parts unsymmetrical dimethylhydrazine – oxidised by dinitrogen tetroxide.

A minute and a half after second stage ignition, the payload fairing separated from around IRNSS-1G at the nose of the rocket.

The second stage concluded its burn about a minute later, separating at four minutes and 22 seconds elapsed time. The solid-fuelled PS3 third stage ignited a little over a second later for a seventy-second burn.

Following third stage burnout the mission entered a coast phase. The spent stage remained attached until shortly before fourth stage ignition – separating eleven minutes and three seconds after liftoff. Ten seconds later the fourth stage’s twin L-2-5 engines ignited for an eight-and-a-half-minute burn. Spacecraft separation occurred twenty minutes and nineteen seconds after launch; thirty-seven seconds after the end of powered flight.

The PSLV C33 launch is the thirty-first consecutive successful launch of a PSLV rocket, which last suffered an anomaly in September 1997 on what was its first launch after being declared operational. On that launch the satellite was placed into a lower-than-planned orbit, however the satellite was still operable and could partially complete its mission.

The only PSLV launch to suffer a complete failure was the rocket’s maiden flight in 1993, when the attitude control system malfunctioned and the vehicle had to be destroyed by range safety.

Thursday’s launch was the third of 2016 for India, with all three launches using PSLV-XL rockets to deploy IRNSS satellites; IRNSS-1E in January and 1F in March. India’s next launch is expected to take place in mid-June with another PSLV-XL deploying the Cartosat-2C imaging satellite along with a host of small satellites.